The present invention relates to multiple input audio systems, for example public address or sound systems which are useful in large meeting halls or in any application where multiple source communication is necessary and where several microphones are used to provide a combined output signal.
Prior arrangements are known where the individual microphone circuits to the output are sound activated by a sound program provided to the individual microphone.
In some arrangements, the output signal level is attenuated by the number of active inputs and in other instances no attenuation is provided. One such arrangement is shown in a booklet entitled "Field Effect Transistor Projects" published by Motorola, Inc. (TM) Phoenix, Ariz. (1966).
Examples of arrangements where the output is modified by the number of active inputs are shown in U.S. Pat. No. 3,814,856 Dugan wherein a sound reinforcement system for a defined area is disclosed having a plurality of input channels and input program sources. In this arrangement, the ambient noise level in the area served by the sound system is sensed and a reference signal is generated in response to the ambient noise level which is used as a reference signal to control the signal proportional to the ambient noise where the combined output signal is then supplied to a comparator for comparison with the reference signal. In this arrangement, a feedback system is provided around the comparator to maintain the access of the input channel to the output. The reference also provides an output system having a variable gain amplifier means where the gain is boosted inversely proportional to the square root of the number of active inputs. While the arrangement is satisfactory in many applications it requires continuous monitoring of the ambient noise level for reference purposes so that such devices are not useful where no accurate ambient noise level can be obtained. Furthermore, devices in accordance with the reference provide an arrangement where the reference signal bears no relation to the number of activated input channels and where localized ambient noise can preclude activation of the input channel.
In this regard, it will be noted that the arrangements in accordance with the reference boost the sum of all the active information channels by an amount proportional to the magnitude of the control signal, regardless of the number of active input channels.
Another prior art arrrangement is shown in U.S. Pat. No. 3,992,584 Dugan, which discloses a sound reinforcement system with a plurality of inputs to an output channel, where the total gain of the system remains constant.
In the arrangement disclosed in U.S. Pat. No. 3,992,584 the individual input channels are actuated by co-action between a reference signal which is proportional to the instantaneous sum of the alternating current signals generated by the active input channels. In addition, the alternating current signal is also filtered and rectified to provide direct current reference signal to be compared with the filtered and rectified output signal of each input channel to selectively activate the input channel.
The use of summed alternating current inputs for a reference signal leads to distortion in the system output as a result of phase cancellation which occurs in the summation of the alternating current reference signals. The distortions are disturbing and adversely affect the effectiveness of the system.
Additionally, since the reference (U.S. Pat. No. 3,992,584) provides an arrangement where each input channel includes a control and where the channel attenuation is achieved by means of a comparative summing amplifier with the summing amplifier as a reference signal, and a direct current signal generated from the output of the microphone, the distortion occasioned by the frequency interference is mixing the signals is significant. This is particularly true since the attenuation of each input channel is responsive to the magnitude of the compared signal as opposed to a switched system. In many applications, the inbalance resulting from the summation of alternating current signals limits responsiveness of the input channel and causes distortion in the system output.
While the foregoing systems have proven satisfactory in certain applications, they present the problems of distortion, or "dead spots", in certain applications and are unsuitable in other applications.
In copending application Ser. No. 4,195, filed Jan. 17, 1979, now abandoned, and arrangement is disclosed providing a multiple source sound amplifying and activation system including at least one first microphone means to generate first sound input signal and at least one second microphone means to generate a second source input signal, at least one input channel means for each microphone means, each input channel means including source input signal filter means to receive a source input signal and transmit a portion of the source input signal as a filtered input signal to full wave rectifier means to provide a full wave rectified direct current channel input signal of selected polarity summing means to receive the direct current input signals from each of the input channel means and to provide a direct current reference signal proportional to the sum of a full wave rectified direct current source input signals received by the summing means input channel comparator means for each input channel to receive and compare the reference signal with the full wave rectified direct current source signal of the input channel and having an output operable to provide a first control signal in response to a first ratio between the reference signal and the direct current channel input source signal and a second control signal in response to a second ratio between the reference signal and the direct current channel input source signal, input signal amplifier means for each input channel to receive the source input signal from the microphone means to transmit an amplified source input signal to output amplification means, where the input signal amplifier means is operable in response to the first control signal and inoperable in response to the second control signal, multiple channel output means including output signal summing means adapted to receive the amplified source input signal from the input signal amplifier means of each activated input channel and attenuator means to receive the output signal from the output signal summing means and having input means to receive the first and second control signals from each of the input channel comparator means of each of the input channels where the attenuator means attenuates the output signal from the output signal summing means in response to the number of first signals received from the input channel comparator means.
In normal operation without compensation, the use of multiple channel inputs to a single output speaker system a disconcerting noise will accompany activation of each channel. To overcome this disadvantage, prior systems have introduced means, for example cooperative resistance, capacitance circuits to slow the rise of a signal level in the output of the newly activated or deactivated input channel. While such arrangements can be adapted to effectively eliminate distortion the benefit is achieved at the sacrifice of responsiveness and such systems can be relatively expensive.